The invention is based on an apparatus for controlling the spring firmness of a motor vehicle, preferably a road vehicle. An apparatus of this kind is known (German Patent No. 16 30 058).
In this known apparatus, two work chambers of a shock absorber, or shock absorber leg, are connected via external lines to a device comprising a pump and two reservoirs. The only valves provided in the connecting lines to the shock absorber are one-way check valves. With an apparatus of this kind, however, the firmness of the shock absorber cannot be varied. To vary the spring firmness, energy must be supplied from outside, via the pump, which can only be accomplished relatively slowly and consumes power.
It is also known, from German Offenlegungsschrift No. 33 04 815, to vary the firmness of a shock absorber by using a separate control means.
In terms of spring and shock absorber firmness, the suspension of present-day vehicles, particularly passenger vehicles, is optimized for an average type of operation. The parameters are structurally fixed and remain unchanged during operation, except for the effects of aging. In extreme operating situations, such as with an empty or a fully loaded vehicle, such vehicles are far from optimally sprung; that is, the suspension is not optimally damped. These situations become still less optimal, the greater the ratio between the weights of an empty vehicle and the same vehicle carrying a maximum load.
A further consideration is that in order to save fuel and to use raw materials economically, the trend in designing new vehicles is increasingly toward a lightweight structure. Yet the usable load attainable will preferably not be reduced. In other words, in the future the above-mentioned weight ratio will become still greater, and it will be increasingly difficult to tune the spring firmness and the shock absorber firmness.